Self-routing packet switching networks such as those using unbuffered banyan switching nodes communicate packets through the switching network on the basis of address information transmitted through the switching network prior to the transmission of the packet. Such a switching network has only one unique path between each input and output pair of the network. A problem with the existence of only one unique route is the effect of unbalanced traffic conditions on the network and failures of switching nodes within the network. The result of unbalance of traffic conditions or a failing node is that it is not possible to route a packet between a given set of input or output pairs of the network. From a maintenance point of view, the existence of only one unique path between each input and output pair results in ease of switch node failure detection since it is possible to send a test packet along any selected path with the network.
One known method for alleviating the reliability and traffic problems in a self-routing network is discussed in the report entitled, "Development of a Voice Funnel System," Bolt, Beranek, and Newman, Inc., Report No. 4098, Aug., 1979, pages III-29 through III-76, which discloses the use of an extra stage of banyan switching nodes at the input of a self-routing network to resolve the previously mentioned problems. The report proposes that this extra stage be identical to other stages of the network and be utilized by adding an extra bit of addressing to the address field of each packet being routed through the switching network. This extra stage of switching would proceed the first routing stage of the network. The extra address bit would be controlled by hardware/software external to the switching network and would determine the route through the switching network. The hardware/software would use this bit so as to avoid a node which was failing or experiencing heavy traffic.
Another method for alleviating these reliability and traffic problems is disclosed in the co-pending application of C. A. Lea, "Multiple Paths in a Self-Routing Packet Switching Network," Ser. No. 654,765. This system utilizes switching nodes which upon receipt of address information select between a multitude of paths through the switching system by the transmission of communication request signals and the receipt of communication available signals from downstream switching nodes. This system avoids the reliability problems by automatically bypassing switching nodes which are experiencing either unbalanced traffic conditions or failing. From a maintenance point of view, a problem does exist in this system since each individual switching node within the path automatically exercises control in order to bypass the failing or overburdened downstream nodes. The result of this decision being made at each individual switching node is that no single control circuit can set up a predetermined path through the network in order to test for failing switched nodes. Hence, it is possible for this system to slowly degrade due to failing nodes without maintenance features being able to detect this degradation except when extremely high traffic conditions are being experienced.